L-Glutamic and L-aspartic acids (so-called excitatory amino acids) are the most common neurotransmitters in the brain and have extremely powerful excitatory effects throughout the central nervous system (CNS). Binding of an excitatory amino acid to an ionotropic receptor causes the opening of a cation-selective channel. The ionotropic receptors are multi-subunit proteins that are further subclassified on the basis of binding to agonists which selectively activate a specific receptor subtype. Such agonists include .alpha.-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA).
Pre-synaptic release of glutamate causes an excitatory post-synaptic current which can be resolved into fast and slow (long-lasting) components. The slow component is mediated by NMDA-gated ion channels and has characteristic features including a slow rise and decay of the current and high Ca.sup.2+ permeability. NMDA receptors are involved in the development of long-term potentiation (LTP), a long-lasting increase in size of the post-synaptic response to a pre-synaptic stimulus of fixed strength. LTP has been proposed as a model for learning and memory (Bliss, T. V. and G. L. Collingridge, Nature (1993) 361:31-39).
NMDA receptors are expressed in virtually every neuronal cell in the body. They are composed of a core subunit that forms a homomeric channel when expressed heterologously. The activity of the core subunit is modified and/or regulated by coexpression of the other subunits. Various cDNAs encoding NMDA channel subunits have been cloned and expressed from rat (Moriyoshi, K. et al. (1991) Nature 354:31-37; Hollmann, M. J. et al. (1993) Neuron 10:943-954) and human (Foldes, R. L. et al. (1993) Gene 131:293-298). Some of the subunit isoforms are encoded by separate genes, whereas others arise by alternative mRNA splicing, and perhaps by RNA editing. Functional diversity can also arise by coexpression of multiple forms of the same subunit in a single cell. The molecular diversity of NMDA receptors has been reviewed by S. Nakanishi (1992; Science 258:597-603).
The NMDA receptor complex must have a binding site for the natural ligand, glutamate, and the cDNA for a glutamate-binding component of an NMDA receptor has been cloned from rat (Kumar, K. N. et al. (1991) Nature 354:70-73). Binding sites for glycine (co-agonist), Zn.sup.2+ (modulatory factor), polyamine (activator), and Mg.sup.2+ (voltage-dependent blocker) have also been identified on the multimeric NMDA channel complex.
High concentrations of extracellular glutamate arise, for example, after stroke or hypoglycemia. Exposure of neurons to a high concentration of glutamate, even for short periods, causes cell death and disease. Excessive activation of NMDA receptors leads to an influx of abnormally large amounts of Ca.sup.2+ into the cell. Toxicity and neuronal death can be mediated by stimulation of downstream pathways which may include activation of gene expression, protein kinases, proteases, and apoptosis (programmed cell death). Neurodegenerative diseases may also be caused by the overexpression and/or inappropriate expression of NMDA receptors.
Discovery of molecules related to the glutamate-binding subunit of the NMDA receptor satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in the diagnosis, prevention, and treatment of conditions and diseases affecting the CNS such as ischemia, hypoglycemia, epilepsy, convulsions, AIDS-related dementia, schizophrenia, chronic neurodegenerative disorders such as Alzheimer's, Huntington's, Creutzfeld-Jacob, and Parkinson's disease, amyotrophic lateral sclerosis, and lathyrism, chronic and neuropathic pain, and defects in motor rhythms, vasomotor tone, baroreception, and LTP, as well as muscle relaxation and sedation.